/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode() : val(0), next(nullptr) {}
 *     ListNode(int x) : val(x), next(nullptr) {}
 *     ListNode(int x, ListNode *next) : val(x), next(next) {}
 * };
 */
class Solution{
public:
  ListNode *deleteDuplicates(ListNode *head){

    set<int> s;
    ListNode *dummyHead = new ListNode;
    dummyHead->next = head;
    ListNode *pre = dummyHead;
    ListNode *p = head;

    while(p != nullptr){
      if(s.count(p->val)){
        pre->next = p->next;
        delete p;
        p = pre->next;
      }else{
        s.insert(p->val);
        pre = pre->next;
        p = p->next;
      }
    }

    return head;

  }
};